STUDY ON THE GEOCHEMICAL CHARACTERISTICS AND SEDIMENTARY TECTONIC EVOLUTION OF THE LONGMAXI FORMATION IN SOUTHERN SICHUAN BASIN CHINA

The ancient sedimentary structure detel mines the source of shale sediments and the sedimentary diagenesis process. In this paper, we systematically studied the element geochemical characteristics of the Longmaxi shale in the southern Sichuan Basin. Combined with the structural background and numerical simulation methods, the evolution of the Longmaxi sedimentary structure was discussed. As a result, the parent rock of the Longmaxi Formation is rich in light rare earth elements, and the distribution of heavy rare earth elements is relatively stable. It tends to gradually increase from bottom to top in the vertical direction, so the tectonic background is at the passive and active continental margins. The La/Sc-Co/Th results show that most of the data points in the southern Sichuan Basin are in felsic rocks, which belong to the origin of an intermediate acid mixture centered on feldspar rocks. And it contains a small amount of medium-sized basic rocks with a delta Eu value of 0,39-1.81, which fluctuates greatly, and the matrix rocks have mixed origin characteristics. The w(Sigma REE)-w(La)/w(Yb) discriminant results show that the lithology of the host rock of the Longmaxi Formation is at the intersection of sedimentary rocks and granites, and has mixed characteristics. The research results of element structure background show that the black rock series of Longmaxi Formation formed in the deep semi-deep sea sedimentary environment of the continental margin is a transitional sedimentary rock series from the continental margin to the deep sea plain. In the structural background recognition results of K2O/Na2O-SiO2 and La-Th-Sc, the tectonic background of the study area is located at the passive and active continental margins. Under the control of the paleo-sedimentary environment, the Longmaxi Formation has gone through four main stages of evolution. The rapid burial stage (about 440 Ma -400 Ma) of the late Indosinian period, the late Hercynian uplift stage (about 400 Ma-270 Ma). the early Yanshanian (about 270 Ma-110 Ma) and the continuous uplift stage after the late Yanshanian (about 110 Ma-10 Ma).